Method and apparatus for compensating for atmospheric distortion

This application is a divisional of prior U.S. Application No. 17/583,421, filed 25 Jan. 2022, titled “Atmospheric Compensation Disc”

The United States Government has ownership rights in this invention. Licensing and technical inquiries may be directed to the Office of Research and Technical Applications, Naval Information Warfare Center Pacific, Code 72120, San Diego, CA, 92152; voice (619) 553-5118; ssc_pac_t2@navy.mil.

Adaptive optics systems that employ LASERs to create artificial guide stars are currently used to actively compensate for atmospheric turbulence. These adaptive optics systems contain imaging systems to measure these guide stars and then use deformable mirrors or spatial light modulators to apply a compensating spatial phase to the propagating light field to correct the atmospheric distortion. Traditional adaptive optics systems, while precise, are inherently expensive, large, complicated, and difficult to implement. There is a need for an improved atmospheric distortion compensator.

The disclosed apparatus below may be described generally, as well as in terms of specific examples and/or specific embodiments. For instances where references are made to detailed examples and/or embodiments, it should be appreciated that any of the underlying principles described are not to be limited to a single embodiment, but may be expanded for use with any of the other apparatus described herein as will be understood by one of ordinary skill in the art unless otherwise stated specifically.

References in the present disclosure to “one embodiment,” “an embodiment,” or any variation thereof, means that a particular element, feature, structure, or characteristic described in connection with the embodiments is included in at least one embodiment. The appearances of the phrases “in one embodiment,” “in some embodiments,” and “in other embodiments” in various places in the present disclosure are not necessarily all referring to the same embodiment or the same set of embodiments.

As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” or any variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or.

Additionally, use of words such as “the,” “a,” or “an” are employed to describe elements and components of the embodiments herein; this is done merely for grammatical reasons and to conform to idiomatic English. This detailed description should be read to include one or at least one, and the singular also includes the plural unless it is clearly indicated otherwise.

is a perspective view illustration of an example embodiment of an atmospheric distortion compensator(hereinafter referred to as compensator) that comprises, consists of, or consists essentially of a discand a rotator. The discis rotationally balanced about a center pointand comprises a phase-modifying structure on a surface. The rotatoris mechanically coupled to the disc's center pointand configured to spin the discabout an axis A. The compensatormay be used to reduce scintillation effects within an electro-optical field of a heterogeneous medium, such as air, which effects are caused by a beamas it propagates through the heterogeneous medium. The compensatoris configured such that the beam, while propagating parallel to the axis A, impinges on the discwhile the discis spinning, thereby allowing the compensatorto control a property of a beamin an effort to reduce scintillation effects.

The beammay be any free space optical beam. Suitable examples of the beaminclude, but are not limited to, LASERs and Gaussian beams. In one example embodiment, the beamis a fully coherent, monochromatic or narrow-band LASER. The compensatorcan be applied to any application where reducing the scintillation at a point in space through turbulence is desired. Common examples would be illuminating an object at range, controlling a targeting beacon, or increasing the signal-to-noise ratio (SNR) of a free space optical communication link. When propagating a normal, fully coherent LASER source through a heterogeneous medium like the atmosphere, the beamwill not have the same spatial intensity distribution at range as it does in the transmit plane. The beam will have peaks and nulls due to the heterogeneous nature of the atmosphere. A common example is when looking at a star at night, it will twinkle. The compensatormay be used to reduce the twinkle of controlled optical beam sources.

The discmay be made of any material capable of supporting the phase-modifying structure. Suitable examples include, but are not limited to, silicon dioxide (SiO2), crown glass, optical borosilicate-crown glass, and super-white soda-lime glass. For example, the Schott Company, of Mainz Germany produces several products from which the discmay be manufactured including, but not limited to, the product B 270®, N-BK7, and N-K5. In one example embodiment, the discis a diffractive optical element made of optical grade glass and the phase-modifying structure comprises a pattern formed in the discthat is controlled by a Gaussian-Schell model of partial coherence to control the propagating beam's spatial coherence property. Other types of patterns are realizable as well. For example, in one embodiment, the phase-modifying structure may be designed to control the propagating beam's optical orbital angular momentum. In one embodiment, the discis transmissive. In another embodiment, the discis reflective.

is a front view illustration, showing the surface, of an embodiment of the dischaving different tracks. In this embodiment, each track applies a different degree of partial coherence to the beam. The discmay have any desired number of separate tracks. The number N of tracks that a discwill have will be dependent on the desired application of the compensatorand will be heavily influenced by the amount of size, weight, and power that the compensatoris able to accommodate. The example embodiment of the discshown incomprises four tracks (,,, and). Three of the tracks,, andare configured to change the phase of the beamto control the partial coherence. Trackis a reference track which may be used for alignment purposes and to enable the beamto be fully coherent if desired. The reference trackmay be configured with a uniform phase profile to not distort the beam. The shadow gradations incorrespond to the phase of the propagating beam. Each trackis configured to apply a different degree of partial coherence to the beam. The tracksare disposed on the discsuch that the beammay be brought into contact with different tracksby moving the discso as to change a distance D between the axis A and the beam(See). The phase-modifying structures on the discmay be realized by selectively varying the thickness T of the disc. In one example embodiment, the phase information in each track is converted to material thickness of the disc, which is entirely dependent on the disc′s index of refraction, the index of refraction of the mediumthe discis spinning in, and the wavelength of the beam. To physically create the varying thicknesses, one can use either subtraction or addition manufacturing techniques. For subtraction, the material could be polished, cut, etched, or vaporized out. Addition manufacturing techniques may utilize lithography techniques to mask and add material in controlled layers, or to utilize 3D printing technology to precisely place material in the necessary locations to build the phase modifying structures.

The rotatormay be any device capable of rotating the disc. Suitable examples of the rotatorinclude, but are not limited to, electric motors, DC-Brushless, stepper motors, AC motors, hand-powered crank, spring drives, water-wheels, and windmills. In one example embodiment, the rotatoris a computer-controlled, variable-speed, electric motor. The rate of rotation of the discmay be altered depending on the desired application of the compensator. For example, for an imaging system application, the discmay have a rotational spatial motion that is at least 30 times greater than the integration time of the imaging system. An example would be, for a one second integration, and a beam width W of 3 millimeters (mm), the discmay have a spatial rotation motion of 90 mm, to achieve 30 independent realizations of a 3 mm beam. This motion would then need to be related to the track information, since each track has a different area and the rotation speed would need to be adjusted accordingly. To keep the math simple, for this example, if the entire trackis 90 mm long, then the discwould need to rotate at 1 revolution per second for a 1 second integration system. If the total tracklength was longer than 90 mm, then the disccould rotate at less than 1 revolution per second, and if the overall trackwas smaller than 90 mm, then the discwould need to rotate faster than 1 revolution per second. Additionally, if the beamis a pulsed LASER, the spin rate of the discmay be tied to the repetition rate of the LASER to ensure independent phase realizations are being imparted on the propagating beam, rather than having the problem of getting stuck in frequency nodes between the spin rate of the discand the repetition rate of the LASER pulse. A classic common example of this problem is imaging airplane propellers that look stationary in the resulting image. This problem/effect should also be considered when establishing discrotation speeds and integration times of a camera in an imaging system application.

is a perspective view of another example embodiment of the compensator. In this embodiment, the compensatorcomprises a linear actuatorand a processor. The processoris operatively coupled to the rotatorand configured to adjust a rotational speed of the disc. In this embodiment, the disccomprises a plurality of N tracks, where each trackis configured to apply a different degree of partial coherence to the beam. The linear actuatoris configured to move the discin a direction that is orthogonal to the beamsuch that the beammay be brought into contact with any desired track. The linear actuatormay be communicatively coupled to the processorthereby allowing the trackthat is interacting with the beamto be changed on the fly. The processormay also be used to control a LASER sourceand the linear actuator. In some embodiments, it may be desirable for the linear actuatorto move the LASER sourcewith respect to the disc.

The linear actuatormay be any device capable of moving the disc in a controlled manner in a direction orthogonal to the axis A. Suitable examples of the linear actuatorinclude, but are not limited to, a pneumatic piston, a hydraulic piston, a linear motor, an electro-mechanical cylinder, a screw-driven linear carriage, a timing-belt-driven carriage, a piezo-electric actuator, a rack-and-pinion driven actuator, a wheel-and-axle actuator, a cam actuator, and a solenoid. The processormay be any computer or logic circuit capable of calculating the rotation speed of the discand position of the linear actuatordepending on the desired application of the compensator. Suitable examples of the processorinclude, but are not limited to, General Purpose Input Output (GPIO) processors, Microcontrollers, Microprocessors, Embedded Processors, Digital Signal Processor (DSP), Media Processors, Application specific integrated circuits (ASICs), Application-Specific System Processors (ASSPs), Application-Specific Instruction Set Processors (ASIPs), Field Programmable Gate Arrays (FPGAs), and General Purpose Computers running Windows, Linux, or MacOS on x86 or x64 architectures.

The compensatormay be used to simply compensate for atmospheric distortion of a propagating optical signal. The compensatormay not be as precise as other, more expensive/comlex adaptive optics systems, but its use will still result in an increased signal to noise ratio (SNR) when compared to propagation without any compensation. This is because the SNR is computed by mean over noise, and if the noise term is significantly reduced with only moderate reductions to the power, the SNR of the entire system will increase.

From the above description of the compensator, it is manifest that various techniques may be used for implementing the concepts of the compensatorwithout departing from the scope of the claims. The described embodiments are to be considered in all respects as illustrative and not restrictive. The method/apparatus disclosed herein may be practiced in the absence of any element that is not specifically claimed and/or disclosed herein. It should also be understood that the compensatoris not limited to the particular embodiments described herein, but is capable of many embodiments without departing from the scope of the claims.